AD9851 datasheet(8/23 Pages) AD

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AD9851

–8–

REV. C

THEORY OF OPERATION AND APPLICATION

The AD9851 uses direct digital synthesis (DDS) technology, in

the form of a numerically-controlled oscillator (NCO), to gen-

erate a frequency/phase-agile sine wave. The digital sine wave is

converted to analog form via an internal 10-bit high speed D/A

converter. An on-board high-speed comparator is provided to

translate the analog sine wave into a low-jitter TTL/CMOS-

compatible output square wave. DDS technology is an innova-

tive circuit architecture that allows fast and precise manipulation

of its output word, under full digital control. DDS also enables

very high resolution in the incremental selection of output fre-

quency. The AD9851 allows an output frequency resolution of

approximately 0.04 Hz at 180 MSPS clock rate with the option of

directly using the reference clock or by engaging the 6

× REFCLK

Multiplier. The AD9851’s output waveform is phase-continu-

ous from one output frequency change to another.

The basic functional block diagram and signal flow of the

AD9851 configured as a clock generator is shown in Figure 11.

The DDS circuitry is basically a digital frequency divider func-

tion whose incremental resolution is determined by the frequency

of the system clock, and N (number of bits in the tuning word).

The phase accumulator is a variable-modulus counter that

increments the number stored in it each time it receives a clock

pulse. When the counter reaches full scale it “wraps around,”

making the phase accumulator’s output phase-continuous. The

frequency tuning word sets the modulus of the counter, which

effectively determines the size of the increment (

∆ Phase) that

will be added to the value in the phase accumulator on the next

clock pulse. The larger the added increment, the faster the

accumulator wraps around, which results in a higher output

frequency.

The AD9851 uses an innovative and proprietary “Angle

Rotation” algorithm that mathematically converts the 14-bit

truncated value of the 32-bit phase accumulator to the 10-bit

quantized amplitude that is passed to the DAC. This unique

algorithm uses a much-reduced ROM look-up table and DSP to

perform this function. This contributes to the small size and

low power dissipation of the AD9851.

The relationship between the output frequency, system clock

and tuning word of the AD9851 is determined by the expression:

fOUT = (

∆ Phase × System Clock)/232

where:

∆Phase = decimal value of 32-bit frequency tuning word.

System Clock = direct input reference clock (in MHz) or 6

× the

input clock (in MHz) if the 6

× REFCLK Multiplier is engaged.

fOUT = frequency of the output signal in MHz.

The digital sine wave output of the DDS core drives the internal

high-speed 10-bit D/A converter that will construct the sine

wave in analog form. This DAC has been optimized for dynamic

performance and low glitch energy, which results in the low

spurious and jitter performance of the AD9851. The DAC can

be operated in either the single-ended, Figures 2 and 8, or dif-

ferential output configuration, Figures 9 and 10. DAC output

current and RSET values are determined using the following

expressions:

IOUT = 39.93/RSET

RSET = 39.93/IOUT

Since the output of the AD9851 is a sampled signal, its output

spectrum follows the Nyquist sampling theorem. Specifically, its

output spectrum contains the fundamental plus aliased signals

(images) that occur at integer multiples of the system clock

frequency

± the selected output frequency. A graphical repre-

sentation of the sampled spectrum, with aliased images, is shown in

Figure 12. Normal usable bandwidth is considered to extend

from dc to 1/2 the system clock.

In the example shown in Figure 12, the system clock is 100 MHz

and the output frequency is set to 20 MHz. As can be seen, the

aliased images are very prominent and of a relatively high energy

CLOCK

OUT

AMPLITUDE/SINE

CONV ALGORITHM

PHASE

ACCUMULATOR

DDS CIRCUITRY

D/A

CONVERTER

COMPARATOR

REFERENCE

CLOCK

TUNING WORD SPECIFIES

OUTPUT FREQUENCY AS A

FRACTION OF REF CLOCK

FREQUENCY

IN DIGITAL

DOMAIN

Figure 11. Basic DDS Block Diagram and Signal Flow of AD9851

120MHz

2ND IMAGE

FOUT

FC+FO

2FC–FO

2FC+FO

3FC–FO

180MHz

3RD IMAGE

220MHz

4TH IMAGE

280MHz

5TH IMAGE

80MHz

1ST IMAGE

20MHz

0Hz

(DC)

FC–FO

SIN (X)/

ENVELOPE

= ( )F/FC

100MHz

SYSTEM CLOCK FREQUENCY

Figure 12. Output Spectrum of a Sampled Sin(X)/X Signal

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